1. Field of the Invention
This invention relates generally to work holders for electrical circuit assemblages and more particularly to automated insertion of the pins of various multiple lead electronic devices into corresponding pin holes in a printed circuit board.
2. Description of the Prior Art
Robots are used for the assembly of electronic equipment such as for mounting multiple lead IC chips and other devices on circuit boards. Examples of such devices include ceramic chip carriers, relays, transformers, IC devices, resistor network devices and other devices having pins or fragile leads which may, for example, be on 0.050 inch centers. The robots require sensor feedback to accomplish such assembly without damage to the devices.
Present known robots use servo-systems and are programmed to move a robotic arm specific distances in the X-Y axes, i.e. in a plane parallel to the board, and, once reaching the proper X-Y coordinates, to then move in the Z axis, i.e. vertically toward and away from the board, to accomplish alignment followed by insertion of the leads into holes in the board. The board is semi-flexible and the periphery of the board is supported. As a result, the board has some compliance in the Z axis. Also, the flexibility of the leads provides additional compliance in the Z direction. Further, Bernoulli type and vacuum type holders, or remote center compliant devices attached to the robotic arm, provide compliance in the X-Y plane while holding the device for insertion. The total X-Y-Z compliance provides beneficial deflection in the assembly system which aids in limiting damage to the devices during insertion attempts.
Insertion forces are set by the compliance of the holders and workpieces. Considering that the 10-100 pound robotic arm is handling workpieces weighing 1-10 grams, the above-mentioned built-in compliance is at times insufficient to limit substantial force overshoot by the robotic arm. As a result, the overshoot may cause excessive insertion forces sufficient to damage the leads in those cases in which the leads do not insert in the holes. Force sensors are used to detect successful assembly. The robotic arm is set to move a certain distance at a certain velocity while monitoring insertion force. The force sensors monitor excessive force and stop arm movement when the force exceeds a programmed limit. The various signal channels used to stop robot motion have limited bandwidth resulting in time delay that causes overshoot and excessive force to be applied.
The foregoing illustrates limitations of the known prior art. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations as set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.